Planetary formation

A stellar striptease

Solar systems may form faster than anyone had expected

THE discs found around a type of young star known as a T-Tauri star are textbook examples of how solar systems are supposed to begin. A rotating cloud of gas and dust starts to contract under its own gravity. It heats up in the middle while the outer regions flatten. As the centre's temperature rises to the point at which nuclear fusion can begin, the dust in the flattening disc coalesces. Eventually, the lumps formed by this coalescence stick together to form planets, and the gas clings to them as atmospheres.

What the textbooks spend fewer pages explaining is that, in most T-Tauri stars, this tidy plot tends to unravel part-way through. It ought to take a lot of time to get from small dust particles to objects the size of Earth, let alone the size of Jupiter. Around 10m years is generally reckoned a reasonable estimate. Unfortunately, most T-Tauri stars seem to have lost their discs by the time they are a mere 3m years old.

These “naked” T-Tauri stars pose a problem for the textbooks. They suggest that planets should be rare, but recent observations indicate that they are not. Now new data presented to the summer meeting of the American Astronomical Society, in Nashville, Tennessee, may help to clear the matter up.

Elizabeth Lada of the University of Florida in Gainesville and Karl Haisch of the University of Michigan in Ann Arbor observed four star clusters, each containing dozens of young stars, in the constellations of Orion and Perseus. Using optical telescopes in Hawaii and Arizona, and a radio telescope in Spain, they measured the amount of infra-red and microwave radiation coming from each star. Such radiation is emitted by warm dust, so a young star that is abnormally bright at these wavelengths is assumed to be surrounded by a dusty disc.

All the stars in a cluster are the same age. So taken together, the observations of these clusters show what kinds of disc, if any, stars of different sizes and ages have. It turns out that almost no discs of the sort thought to produce planets are found in clusters more than 3m years old.

Dr Lada is quite certain that these discs are not there. The observations were so sensitive that even at a distance of 1,000 light years an asteroid's-worth of dust would show up in them. On the face of it, that is bad news for theoreticians. If people want to believe that planets form routinely around T-Tauri stars, she concludes, they will have to think of ways to allow this to happen in 3m years instead of 10m.

Jeff Bary and David Weintraub, of Vanderbilt University in Nashville, are not so sure. The observation that the fine dust disappears rapidly is, they say, no reason to think that the manufacture of planets must be a 3m-year rush job. All that is necessary is a partial consolidation into somewhat larger objects—the sorts of object that might be expected to form as intermediaries in the planet-building process.

If this idea is right, then the discs of older T-Tauri stars still contain not only most of their original solid matter, but also most of the accompanying gas (mainly molecular hydrogen). Of itself, this gas would not emit much infra-red radiation. But if suitably excited, it might.

Such excitement could come in the form of X-rays. According to Joel Kastner of the Rochester Institute of Technology in New York state, these could be generated in one of two ways. Dr Kastner has studied X-ray emissions from T-Tauri stars using a satellite called Chandra. Some X-rays emanate from matter that falls from the disc towards the star itself. But X-rays are also emitted by giant magnetic short-circuits in the outer layers of the disc—large versions of the flares in the sun's corona.

Whatever the source, such X-rays will cause hydrogen molecules to vibrate, and these vibrations will be released as light of a specific wavelength. Mr Bary and Dr Weintraub used a telescope at Kitt Peak in Arizona to look for light of this wavelength around a naked T-Tauri star. And they found it. Naked stars, it seems, are not naked after all. They are just dressed in a diaphanous incipient solar system.